High-pressure gaseous discharge lamp including a starting electrode

ABSTRACT

Ignition of the arc between the main electrodes of a highpressure gaseous discharge lamp is achieved by placing an integral thermionic starting electrode adjacent one of the main electrodes and then concurrently heating the starting electrode and applying a low-voltage starting voltage to the starting electrode and the adjacent main electrode. After the resulting auxiliary arc has heated the adjacent main electrode, the operating voltage is applied across the main electrodes and the starting voltage is terminated, thus causing the arc to shift from the thermionic electrode to the other main electrode. The starting electrode is heated electrically by connecting it to an external power source through a pair of leads sealed through the lamp envelope.

United States Patent Daniel A. Larson [72] Inventor 1,959,419 5/l934 Freedman 315/336 X Cedar Grove, NJ. 2,474,403 6/1949 Reichinstein 315/335 [21] P 9379 Primary Examiner-Ronald L. Wibert [22] Filed Feb. 6, 1970 Assistant Examiner Orville B. Chew, ll {45] panimed 1972 Attorneys-A. T. Stratton W. D. Palmer and D. S. Buleza '[73] Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

ABSTRACT: Ignition of the arc between the main electrodes 54 HIGH-PRESSURE GASEOUS DISCHARGE LAMP 9 a 'g f i f l '3 aclgeved INCLUDING A STARTING ELECTRODE mg an m egra ermtonlc 5 ar mg e co ro e a acen oneo 2 CI 2 D the main electrodes and then concurrently heating the starting aims rawmg electrode and applying a low-voltage starting voltage to the [52] US. Cl 313/198, starting electrode and the adjacent main electrode. After the 3 l5/330 resulting auxiliary arc has heated the adjacent main electrode, [51] Int. Cl H0lj 61/54 the operating voltage is applied across the main electrodes and [50] Field of Search 315/168, the starting voltage is terminated, thus causing the arc to shift 330, 335, 336; 313/197, 198 from the thermionic electrode to the other main electrode. The starting electrode is heated electrically by connecting it to [56] References cued an external power source through a pair of leads sealed UNlTED STATES PATENTS through the lamp envelope. 1,230,004 6/1917 Meikle 315/335 X FT.- 'F i a: I l 1' l 1' i a i' I. MAIN 3 VOLTAGE SUPPLY HEATER 2 I VOLTAGE 34 SOURCE 35 I4"- AUXIJARY VOLTAGE SUPPLY 20;

n i ii i H i t f ES 1 I 1, Lyea 24 LIN-t I 37 l PATENTEU JAN] 1 I372 HEATER VOLTAGE SOURCE INVENTOR Daniel A. Larson [35 AUXILIARY VOLTAGE SUPPLY FIG.

MAIN VOLTAGE SUPPLY WITNESSES BACKGROUND OF THE INVENTION 1. Field of'the Invention The presentinvention relates to electric lamps and has particular reference to an improved high-pressure gaseous discharge lamp and apparatus and to a novel method of reliably starting such lamps on a low-voltage power supply.

2. Description of the Prior Art As is well known in the art, high-pressure gaseous discharge lamps require a starting voltage which is much higher than that required to maintain the arc and keep the lamp operating. A 2,500 watt short-arc lamp containing xenon at a pressure of about 8 atmospheres and having a nominal operating voltage pulse of 30 volts DC, for example, requires a starting voltage pulse of approximately 30,000 volts to initiate the arc and start lamp. Hence, the starting circuits conventionally employed to start such lamps are complicated, cumbersome and expensive.

Various prior art lamp designs utilizing an auxiliary electrode located near the main electrodes have been developed to reduce the starting voltage and simplify the associated circuitry. A short-arc lamp having a starting electrode that is connected to a pulse source and a transformer which apply a high-starting voltage between the starting electrode and the lamp cathode is disclosed in U.S. Pat. No. 3,403,293 issued Sept. 24, 1968 to C. H. Michelson. Another type of short-arc lamp having an auxiliary probelike electrode that is capacitively coupled to an exterior pulse-generating means is disclosed in U.S. Pat. No. 2,727,187 issued Dec. 13, 1955 to A. A. Webb et al. While these prior art lamp designs reduce the required starting voltage to some extent, they still require a voltage pulse of rather high magnitude and a costly pulsegenerating circuit.

High-pressure gaseous discharge lamps constructed to permit starting on low voltage by temporarily shorting the main electrodes and then magnetically separating them to trigger the are are also known but are not entirely satisfactory since they are expensive to make and present the potential danger of allowing the electrodes to become welded to one another. In addition, the envelope arm which contains the mechanism for magnetically moving the electrode must be lengthened and extreme care must be observed in assembling the various movable components of the mechanism to insure reliable starting.

A short-arc lamp which is started on low voltage by a magnetically actuated striker bar is disclosed in U.S. Pat. No. 3,452,236 issued June 24, 1969 to N. C. Beese.

SUMMARY OF THE INVENTION It has been found that the foregoing disadvantages and problems associated with the prior art devices can be eliminated by employing a thermionically emissive component as the starting electrode and heating it during the starting cycle. The thermionic electrode is placed adjacent one of the main electrodes and, in response to starting voltage of low magnitude that is applied across the starting electrode and adjacent main electrode, initiates an are which is subsequently transferred to the other main electrode when the operating voltage is applied to the lamp terminals.

In accordance with a preferred embodiment of the invention, the starting electrode comprises a tungsten wire coil that is coated with an electron-emissive material and is heated to emitting temperature by connecting it to an external power source of a few volts through a pair of lead wires that are sealed through the envelope. The starting voltage is applied to the thermionic trigger electrode and the adjacent main electrode by an inexpensive starting circuit and the arc is transferred to the other main electrode by connecting the main electrodes to the Operating voltage source after the gas has been ionized and the arc has been struck.

BRIEF DESCRIPTION OF THE DRAWINGS A'better understanding of the invention will be obtained by referring to the exemplary embodiment shown in the accompanying drawing, wherein:

FIG. 1 is an elevational view, partly in section, of an improved short-arc lamp and a schematic diagram of the various associated circuits constructed in accordance with the present invention; and

FIG. 2 is an enlarged perspective view of the thermionic starting electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 there is shown a 2,500 watt high-pressure shortare lamp 10 which embodies one form of the present invention and consists of an elliptical envelope 12 that is composed of quartz or another suitable high-temperature light-transmitting material and is terminated by oppositely disposed sealed arms 13 and 14. The envelope 12 contains a suitable ionizable gas, such as xenon at a pressure of about 8 atmospheres, and an anode l6 and a cathode 18 that are anchored in the envelope arms 13 and 14, respectively. The anode 16 is connected to a plurality of ribbon conductors 17 that are embedded in the arm 13 of fused quartz and are connected to a tungsten rod 19 that is anchored in and extends beyond the outer end of the arm 13. A metal cap 21 is fastened to the rod 19 and to the outer end of the arm 13 and serves as one of the lamp ter minals. An identical lead-in conductor assembly, consisting of ribbon conductors 20 and a tungsten rod 22 that are embedded in the other envelope arm 14, connects the cathode 18 to another attached metal cap 24 which constitutes the other terminal of the lamp 10.

As will be noted in FIG. 1, the cathode 18 is tapered and smaller than the anode 16 and has its shank portion 'protectively enclosed in a quartz sleeve 25 that is fused with the arm 14. The anode 16 and cathode 18 are fabricated from solid tungsten and are anchored in the envelope arms 13 and 14 and connected to the respective lead-in conductor assemblies in accordance with standard lamp-making techniques.

Ionization of the supraatmospheric gas within the lamp l0 and ignition of the arc is achieved in accordance with the present invention by utilizing a thermionically emissive starting electrode 26 that is disposed adjacent one of the main electrodes 16 and 18 and outside the arc path defined by the main electrodes. The starting electrode 26 comprises a refractory metal member that is coated with electron-emission material and, in the illustratedembodiment, consists of a tungsten wire coil of a few turns that is coated with a mixture of alkaIine-earth metal oxides.

As shown more particularly in FIG. 2, the starting electrode 26 comprises a tungsten core wire 27 that is overwound with a fine tungsten wire 29 to form a composite wire which, in turn, is wound into a helix of a few turns 30. The turns of the helix are coated with a layer 31 of emission material that covers and is held in place by the winding 29 of fine wire. The emission material preferably consists of a mixture of calcium oxide, barium oxide and strontium oxide of the type used on the cathodes of fluorescent lamps. The emission coating initially contains the alkaline-earth constituents in the form of carbonates and the latter are decomposed and converted into oxide form during the bulb-lehring and exhaust operations in the well-known manner.

In the case of a short-arc discharge lamp low-voltage the type shown in FIG. 1 that is adapted to be operated on direct current, the starting electrode 26 is preferably disposed adjacent the cathode 18 since it is smaller and will heat up faster than the anode 16. The starting electrode 26 is supported in the desired position by a pair of spaced rigid lead-in conductors 28 then are protectively enclosed in a quartz sleeve 32 and are sealed through the wall of the envelope 12. The outer ends of the lead-in conductors 28 are connected to a suitable low-voltage heater" source 33 by a pair of conductors 34 and a switch 8,. One of the conductors 28 is also connected to a starting circuit comprising a low-voltage auxiliary power source 35, a suitable ballast such as a resistor 36, and a switch 8 that are connected in series with each other and the terminal cap 24 by conductors 37 and 38.

Operation of the lamp 10 and the application of the normal operating voltage across the main electrodes 16 and 18 is accomplished by connecting the lamp terminal 24 to one side of the main voltage supply 39 with a conductor 40 and connecting the other lamp terminal 21 to the other side of the main voltage supply with another conductor 41 through a series connected switch S and a suitable main ballast such as another resistor 42.

To start the lamp 10, switches S and 8;, are opened and switch S is closed, thus connecting the starting electrode 26 to the heater power source and passing an electric current through the tungsten coil which heats the coating of electronemissive material. When the starting electrode 26 has reached electron-emitting temperature, switch S is closed and a starting voltage of sufiicient magnitude in the order of 50 to 100 volts DC depending upon the gas pressure and the spacing between the cathode 18 and starting electrode 26, is applied across the latter by the starting circuit to ionize the gas and trigger an arc between the cathode 18 and starting electrode 26. The switches are shown in the starting phase position in FIG. 1.

After the cathode 18 has been heated sufficiently by the arc, switch S is closed and switch S is opened, thus applying the main voltage across the main electrodes 16 and 18 which initially would be slightly higher than the normal operating voltage in order to initiate break down between the main electrodes. The are then transfers to the main electrodes 16 and 18 and the lamp 10 operates in the normal fashion on an operating voltage and current that is controlled by the ballast 42. The switch 5,, of course, is then opened to disconnect the starting electrode 26 from its heater power source 33. The foregoing sequence of operations can be performed automatically, if desired, by properly designed and correlated timedelayed relays or the like.

TEST DATA Tests have shown that breakdown is achieved in 8 atmospheres of xenon with a starting electrode spaced 8 millimeters from the adjacent main electrode using a starting potential of 100 volts DC. The starting electrode in this case consisted of a tungsten wire coil that had seven turns, a barrel length of about 7 millimeters, a barrel diameter of millimeters and was wound from a core wire approximately 0.4 millimeters in diameter and overwound with a fine wire of about 0.1 mm. diameter. The tungsten coil was heated to electronemitting temperature by connecting it to a 5 volt DC voltage source.

The aforesaid tungsten starting coil was coated with an emission coating consisting of a mixture of approximately 37 percent strontium carbonate, 13 percent calcium carbonate and 50 percent barium carbonate that was applied to the coil in the form of a thick slurry and then heated during lamp exhaust to break the carbonates down into the corresponding oxides.

Other tests have shown that a starting electrode of the same type described above will strike an arc in 1 atmosphere of argon when the starting electrode is spaced 5 millimeters from the cathode and a starting potential of 50 volts AC (r.m.s.) is applied to the cathode and starting electrode and that breakdown between the main electrodes will occur when a potential of about 50 volts DC is applied to the main electrodes. ionization and ignition of the arc did not occur between the main electrodes at 700 volts when the starting electrode was not used.

Other types of coils can, of course, be used as the starting electrode and their design parameters will vary dependent upon the size of the adjacent main electrode since the thermal mass of the latter controls the amount of starting current which the coil must withstand.

It will be apparent from the foregoing that an improved high-pressure gaseous discharge lamp and apparatus has been provided which can be reliably started on a low-voltage power source by a thermionic starting electrode that can be easily built into the lamp and reduces the size and cost of the starting circuit and power supply.

I claim as my invention:

1. A high-pressure gaseous discharge short-arc lamp comprising, in combination;

a sealed light-transmitting envelope containing an ionizable gas at a pressure of at least one atmosphere;

a pair of main electrode within said envelope comprising a cathode and an oppositely disposed enlarged anode that are spaced to sustain an electric discharge therebetween when the lamp is energized and said main electrodes are heated by said discharge,

independent lead-in conductor means connected to said cathode and anode and sealed through said envelope,

means for initiating an electric discharge within said envelope comprising a starting electrode that is located adjacent and to one side of said cathode, said starting electrode comprising a coiled core wire that has an overwinding of fine wire thereon and is coated with an electronemissive material which includes alkaline-earth metal oxides, and

means for passing an electric current through said starting electrode and thereby heating the coating thereon to electron-emitting temperature comprising a pair of leadin conductors that are connected to said starting electrode and sealed through said envelope.

2. The short-arc lamp of claim 1 wherein;

said core wire and fine overwound wire are composed of tungsten,

said ionizable gas comprises xenon at a pressure of at least 5 atmospheres, and

said electron-emissive coating comprises admixed barium oxide, calcium oxide and strontium oxide. 

1. A high-pressure gaseous discharge short-arc lamp comprising, in combination; a sealed light-transmitting envelope containing an ionizable gas at a pressure of at least one atmosphere; a pair of main electrode within said envelope comprising a cathode and an oppositely disposed enlarged anode that are spaced to sustain an electric discharge therebetween when the lamp is energized and said main electrodes are heated by said discharge, independent lead-in conductor means connected to said cathode and anode and sealed through said envelope, means for initiating an electric discharge within said envelope comprising a starting electrode that is located adjacent and to one side of said cathode, said starting electrode comprising a coiled core wire that has an overwinding of fine wire thereon and is coated with an electron-emissive material which includes alkaline-earth metal oxides, and means for passing an electric current through said starting electrode and thereby heating the coating thereon to electronemitting temperature comprising a pair of lead-in conductors that are connected to said starting electrode and sealed through said envelope.
 2. The short-arc lamp of claim 1 wherein; said core wire and fine overwound wire are composed of tungsten, said ionizable gas comprises xenon at a pressure of at least 5 atmospheres, and said electron-emissive coating comprises admixed barium oxide, calcium oxide and strontium oxide. 